The lseek(2) family of functions reposition the offset of the open
file associated with the file descriptor fd to offset bytes relative
to the start, current position, or end of the file, when whence has
the value SEEK_SET, SEEK_CUR, or SEEK_END, respectively.
For more details, return value, and errors, see lseek(2).
Four interfaces are available: lseek(2), lseek64(), llseek(2), and
_llseek(2).
lseek()
Prototype:
off_t lseek(int fd, off_t offset, int whence);lseek(2) uses the type off_t. This is a 32-bit signed type on 32-bit
architectures, unless one compiles with
#define _FILE_OFFSET_BITS 64
in which case it is a 64-bit signed type.
lseek64()
Prototype:
off64_t lseek64(int fd, off64_t offset, int whence);
The library routine lseek64() uses a 64-bit type even when off_t is a
32-bit type. Its prototype (and the type off64_t) is available only
when one compiles with
#define _LARGEFILE64_SOURCE
The function lseek64() is available since glibc 2.1, and is defined
to be an alias for llseek().
llseek()
Prototype:
loff_t llseek(int fd, loff_t offset, int whence);
The type loff_t is a 64-bit signed type. The library routine
llseek() is available in glibc and works without special defines.
However, the glibc headers do not provide a prototype. Users should
add the above prototype, or something equivalent, to their own
source. When users complained about data loss caused by a miscompi‐
lation of e2fsck(8), glibc 2.1.3 added the link-time warning
"the `llseek´ function may be dangerous; use `lseek64´ instead."
This makes this function unusable if one desires a warning-free com‐
pilation.
_llseek()
On 32-bit architectures, this is the system call that is used to
implement all of the above functions. The prototype is:
int _llseek(int fd, off_t offset_hi, off_t offset_lo,loff_t *result, int whence);
For more details, see llseek(2).
64-bit systems don't need an _llseek() system call. Instead, they
have an lseek(2) system call that supports 64-bit file offsets.

This page is part of release 5.00 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 LSEEK64(3)